Swine vesicular disease is a viral disease by contact infection and its vesicular pathology is essentially similar to foot-and-mouth disease in anatomy and histology. In addition to vesicular pathology, any pathology during autopsy cannot be found in other tissues. The vesicles generally occur in crown belt, the area between toe and hoof, nose, mucous-membrane of oral cavity and tongue. Swine vesicular disease is similar to foot-and-mouth disease in clinical conditions. Although a swine contracting swine vesicular disease will not die, the infected swine will have the conditions including pain, creep and delayed growth and development. Since swine vesicular disease is very similar to foot-and-mouth disease in clinical conditions, it is easily to cause a mistaken examination to affect the prevention of foot-and-mouth disease, which is a highly acute and infectious disease of livestock and wild cloven-hoofed animals and deeply strike the livestock industry; therefore, the countries having progress in livestock industry regard foot-and-mouth disease as the first preventive object. To effectively control foot-and-mouth disease, the developed countries regard swine vesicular disease as malignant infectious disease and adopt cleaning policy, i.e., the extermination of the swines contracting swine vesicular disease. Further, these countries strictly control imported pork and prevent the swines contracting swine vesicular disease into the countries.
The causative agent of swine vesicular disease is swine vesicular disease virus (SVDV). SVDV belongs to the genus enterovirus of the Picornaviridae. The genome of SVDV is a single-stranded RNA with a positive polarity, 7400 nucleotides long and consists of P-1, P-2 and P-3 regions. All viral genes only have one open reading frame and can synthesize a large polyprotein. The polyprotein can be cleaved by virus-specific proteinase to form a mature viral protein. The polyprotein of P-1 region includes four capsid proteins, i.e., VP1, VP2, VP3 and VP4 (Toru Inoue et al, J. gen. Virology (1989), 70, 919-934). The epitope of SVDV is mainly on amino acid residues 87, 88, 272 and 275 of VP1; 70, 154, 163 and 233 of VP2; 60, 73 and 76 of VP3 (Toru Kanno et al, J. gen. Virology (1995), 76, 3099-3106). The polyprotein of P-2 region includes 3 proteins: 2A, 2B and 2C wherein 2A is a protein cleaving enzyme. The polyprotein of P-3 region includes 4 proteins: 3A, 3B, 3C and 3D; wherein 3A is a protein-cleaving enzyme and 3D is an RNA polymerase.
As previously reported, the genes of SVDV only have 7400 base pairs and encode 11 proteins. Each protein is essential for viral survival. Up to now, none of the prior art discloses that the deletion of any fragments of viral protein will not affect the viral survival.
The known papers have disclosed all cDNA sequences of SVDV strains H/3' 76 (Toru Inoue et al, J. gen. Virology (1989), 70, 919-934), J 1' 73 (Toru Inoue et al, Nucleic Acids Research, 1993, Vol. 21. No.16, 3896) and UKG/27/72 (P. Seechurn et al. Virus Research 16, 255-274 (1990)). The homology among the strains is larger than 98%.
Furthermore, the homology between coxsackievirus B5 and the capsid protein of swine vesicular virus is 92-96% but between coxsackievirus 2A and the capsid protein of swine vesicular virus is 86.7-88%. Although the clinical conditions of foot-and-mouth disease are very similar to those of swine vesicular disease, the genetic homology between foot-and-mouth disease virus (FMDV) and SVDV is lowered to approximately 25%. The conventional diagnosis and identification of FMDV and SVDV are depended on serological assay. However, since the technique of DNA polymerase chain reaction is greatly advanced in detecting DNA sequence in a sample and is easier and quicker than serological assay, the samples having similar sequences can be differentiated according to appropriate primer design. Therefore, it has been reported that the use of polymerase chain reaction to diagnosis and identify the viruses having similar clinical conditions or capsid proteins (Otfried Marquardt et al. J. Virological Methods 53(1995) 189-199, S. Zientara et al. J. Virological Methods 53(1995) 47-54 and Arch Virol (1996) 141:331-344). Due to the progress of PCR technique, especially directed to the improvement of the quantitative PCR technique, the methods by PCR technique for examining swine vesicular disease, coxsackie disease and foot-and-mouth disease will be more and more widespread.
In 1981, Baltimore cloned cDNA of full-length poliovirus, which belongs to Picornaviridae, to expression vector of mammalian cells containing SV40 promoter, and then transfected HeLa cells with the vector to produce infectious poliovirus (Baltimore et. al. Science 214, 916-919 (1981)). Thus, any person skilled in the art can modify cDNA sequence of RNA virus in expression plasmids and then transfected suitable host cells with the plasmids to produce mutagenic RNA virus. Therefore, the genetic recombination of DNA virus can be easily and rapidly operated in DNA level. Thereafter, the strains mutated on epitopes of poliovirus have been made and the vectors for use in polyvalent vaccine have been developed (Paul Andino et al. Science 265 (1994) 1448-1451; Tae-Jin Yim et al. Virology 218, 61-70 (1996)).
In 1990, Toru Inoue cloned cDNA of SVDV(Japan H/3' 76 strain), which belongs to Picornaviridae, to expression plasmid pSVL of mammalian cells containing SV40 promoter, and then transfected IBRS-2 cells with the plasmid to produce SVDV(Toru Inoue et al J. gen. Virology (1990), 71, 1835-1838). However, the viral plaques formed by the recovered virus are smatter than those of parental virus (H/3' 76 strain). It may be resulted from mutagenesis during the cloning in complex expression plasmids (through about 10 times cloning steps).
In current livestock industry, there is no vaccine having good immunity for swine vesicular disease and differentiating from wild type strain of SVDV to prevent swine vesicular disease. Therefore, it urgently requires a vaccine having mutant strains capable of differentiating from wild type strain of SVDV to immunize swines to decrease economic loss.